I. Iron metabolism and iron deficiency anemia

A. Iron and its metabolism

The fourth most abundant element in the earth's crust, iron is only a trace element in biologic systems, making up only 0.004% of the body's mass. Yet it is an essential component or cofactor of numerous metabolic reactions. By weight, the great proportion of the body's iron is dedicated to its essential role as a structural component of hemoglobin. Hemoglobin without iron is totally useless (in fact, hemoglobin with Fe+++ instead of the normal Fe++ is the ugly brown methemoglobin and is also worthless as an oxygen carrier). Without sufficient iron available to the rbc precursors, normal erythropoiesis cannot take place, and anemia develops. On the other hand, iron is a toxic substance. Too much iron accumulating in vital structures (especially the heart, pancreas, and liver) produces a potentially fatal condition, hemochromatosis. Clearly, iron, like oxygen, is another of the deleterious substances that evolution has led biologic systems into flirtation with.

Most of the iron not circulating in the rbc's is stored in the Fe+++ (ferric) oxidation state. This iron is stored in marrow histiocytes in the form of hemosiderin. When iron is needed by the erythron, the hemosiderin gives up its iron to nearby rbc precursors who line up around the histiocyte like pigs around a trough. Hemosiderin is easy to see microscopically in smear or section preparations of marrow, due to the ferric iron's ability to produce an intense blue color in the Prussian blue stain. This reaction is the basis of the routine "iron stain" done on bone marrow specimensto assess adequacy of depot iron. Erythrocytes would not be expected to stain positively, since they contain ferrous iron. Because the body is dealing with such an essential but dangerous and biologically rare substance (and because you have become resigned to endless memorization in your hazing as medical students), you would expect that there would be some kind of complicated mechanism for the absorption and transport of iron.

Iron is present in greatest concentration in meat and dark green vegetables. The U.S. Recommended Daily Allowance for adults is 10 mg for males, 18 mg for menstruating females. The average daily American diet contains about 10 mg iron, of which only about 1 mg is absorbed. What goes in must come out, and in the adult male, the 1 mg/day iron loss occurs almost exclusively in the stool. For reproductive-aged females, an additional route is the menstrual flux, which accounts for a wildly variable incremental loss. While the average monthly menstrual blood loss is 40 mL (equivalent to 16 mg iron), some women who consider themselves healthy may lose up to 495 mL blood (about 200 mg iron) per menstrual period, or an average of about 7 mg iron per day (200 mg iron ÷ 28 days/cycle). It is not surprising that iron deficiency anemia is relatively common in women of this age group.

Following ingestion, iron is absorbed primarily in the duodenum, although any portion of the small bowel is efficient at iron absorption (in contrast to the situation with B12, as noted below). Only ferrous iron can be absorbed. The normal gastric acidity provides an optimal environment for the reduction of any ferric iron to the ferrous version. In states of iron depletion, a greater proportion of iron is absorbed than in states of normal iron depots. After uptake, the ferrous iron is transported to the subepithelial capillaries (possibly by intracellular transferrin), and released into the bloodstream. There it is oxidized to Fe+++ and again taken up by plasma transferrin. It is then conveyed to the erythron (and reduced again to the ferrous version) or to marrow histiocytes for eventual incorporation into hemoglobin. Storage iron exists as part of a ferric iron-apoprotein complex called ferritin. Ferritin molecules are water-soluble and are present in plasma in concentration equilibrium with ferritin molecules in histiocytes. Therefore, decreased iron stores (as is seen in impending iron deficiency anemia) are reflected by decreased serum concentration of ferritin, a substance easily measured in clinical laboratories. In marrow histiocytes, most of the ferritin molecules glom up into visible (through the microscope, that is) blobs of cytoplasmic inclusions rich in iron and poor in apoprotein; this substance is called hemosiderin. Hemosiderin is easily seen with the Prussian blue stain but can even be observed in unstained preparations of marrow, if present in sufficient quantities, due to the natural golden brown color of iron itself. Since hemosiderin is not soluble, it does not float around in the plasma with ferritin.

B. Iron deficiency anemia

When there is insufficient iron available for the normal production of hemoglobin, anemia results. The cells which are produced are small and pale, and indices from such specimens show low values for MCHC and MCV. Therefore, the classic anemia that occurs in iron deficiency is hypochromic, microcytic. Early or mild cases of iron deficiency anemia (IDA) show microcytosis without hypochromia. Since this is a hyporegenerative anemia, the retic count would be expected to be low; however, because so many cases of IDA are due to chronic bleeding, it is not uncommon to see patients with episodes of hemorrhage that have produced an elevated RPI on clinical presentation. It would appear that the marrow is able to produce a transient response to bleeding, but over the long haul it is a day late and a dollar short. Another finding commonly seen on clinical presentation is thrombocytosis, again probably reflecting marrow response to bleeding. The sine qua non of IDA is the observation that there is essentailly no iron in the marrow (that's zero, zilch, nada), since erythropoiesis can occur normally as long as at least some storage iron is present. In iron-deficient states, one of the body's clever reactive phenomena is the increase in production of transferrin. This is sometimes measured as total iron binding capacity of serum (TIBC). Without the availability if iron, a heme precursor, protoporphyrin, and a porphyrin side-reactant, zinc protoporphyrin, accumulate in the red cell. These may also be measured. In summary, the laboratory features of IDA are:

Hypochromic, microcytic anemia

Variable retic count

Increased erythrocyte zinc protoporphyrin

Increased free erythrocyte protoporphyrin

Decreased serum iron

Increased TIBC

Decreased serum ferritin

Absent marrow storage iron

Variable platelet count

C. Causes of IDA

Iron stores can be depleted either through insufficient intake or excessive loss. In America, the combination of our meat-rich diet and fortification of our staples (such as Wonder Bread, Quaker Instant Grits, and Kellogg's 40% Bran Flakes) with added iron makes dietary insufficiency a very rare condition. The one exception to this is the case with milk-fed infants. Bovine milk has almost no iron. An iron-deficient state in such babies often is sown in the fertile soil of an antenatal life in a mother who was also overtly or borderline iron-deficient (iron requirements are markedly increased in pregnancy due to the demands of developing the fetal tissues). Fortunately most infant formulas are fortified with iron now. Moreover, today's parents are so paranoid about iron deficiency that it is surprising that the typical child of the 1990's can get through an airport without setting off any alarms. Still, nutritional cases of IDA do occur.

Although dietary deficiency of iron is rare, individuals with gastrointestinal lesions producing malabsorption syndromes may fail to assimilate sufficient iron to maintain the erythron, even in the face of adequate iron intake.

The much more important cause of iron depletion is chronic blood loss. In females, this is usually due to menses. Other more sinister causes include chronically bleeding lesions of the gastrointestinal tract, from reflux esophagitis, to peptic ulcers, to gastric or colorectal adenocarcinomas. Because these bad guys may be lurking asymptomatically, spilling erythrocytes here and there for months, all cases of iron deficiency anemia must be thoroughly investigated for the presence of bleeding sites. This is especially true in cases involving females who are not of reproductive age and in all males. In these demographic groups, to simply treat IDA with iron and not investigate for bleeding lesions is unequivocal gross negligence.

D. Treatment of IDA

Oral iron preparations are available for treatment of theses cases. The cheapest and probably best absorbed is non-coated ferrous sulfate (FeSO4), although many other products are satisfactory. It is important to specify on the prescription that non-enteric coated preparations be used, so as to maximize iron availability for absorption. Since acute iron overdose is a potentially fatal toxicosis, iron tablets should be kept out of reach of children.

In certain cases, such as in gastrointestinal malabsorption syndromes, it may be necessary to give parenteral iron. This preparation, iron dextran (Imferon®), may be given IM or IV. Since it may produce anaphylactic shock, it needs to be given under direct physician supervision. Transfusion, which immediately restores all iron stores to surfeit, is very dangerous in chronically anemic patients because of the demand the increment in blood volume puts upon the already taxed heart. It is almost never indicated in iron deficiency anemia.

II. Anemia of Chronic Disease (ACD)

This is a condition seen in individuals suffering from chronic infections, noninfectious inflammatory diseases (such as rheumatoid arthritis), and neoplasms. The following pathogenetic observations have been made to help characterize the anemia:

Decreased rbc life span. This appears to be due to a factor or factors extrinsic to the red cell. The chemical nature of such factor(s) is completely unknown.

Impaired iron metabolism. Iron accumulates in the marrow histiocytes, but its uptake into rbc precursors is impaired. Therefore the marrow shows decreased sideroblastic iron in the face of increased histiocytic iron. This is probably because lactoferrin (an iron-containing compound made by neutrophils to employ in destroying bacteria) competes with transferrin for surface receptors on macrophages. The iron in lactoferrin is not available for use by developing red cell precursors.

Refractoriness to erythropoietin is an effect of lymphokines that are secreted by turned-on immune cells. This anti-growth effect of inflammation is not limited to erythroblasts; even hair and nails grow more slowly in times of inflammation.

The anemia is usually said to be normochromic/normocytic, but most patients actually have a slightly decreased MCHC (thus hypochromia). A minority of patients will be microcytic as well. The serum iron is decreased, as is the transferrin (or TIBC) in contrast to iron deficiency anemia, where transferrin is elevated. The absolute retic count is normal or slightly elevated. Bone marrow biopsy shows increased histiocytic iron and decreased sideroblastic iron, but no other morphologic findings are characteristic of this condition.

III. Megaloblastic anemias

These are a number of conditions which have in common the failure to synthesize adequate amounts of normal DNA. The anemias are macrocytic, since hemoglobinization is allowed, but cells mature more slowly in the marrow; therefore, the cells vegetate in the marrow, slowly maturing but stuffing their greedy little figurative mouths with iron, making hemoglobin, and getting larger as a result. Although some of these obese cells make it out of the marrow, many more never mature properly and eventually are destroyed before they have tasted the thrill of the extramedullary hunt. This phenomenon is referred to as ineffective erythropoiesis. Such marrows are packed with erythroid precursors, even in the face of severe anemia. The rbc precursors are notable morphologically for their immature, sometimes even blast-like chromatin in large nuclei. Such cells are called megaloblasts. Megaloblastic changes are not limited to the erythroid precursors, but are also seen in myeloid precursors. In some cases of megaloblastic anemia, there is concomitant leucopenia and thrombocytopenia, reflecting the troubled development of granulocytes and platelets as well.

A. Pathogenesis of megaloblastic anemias

The megaloblastic state results from an imbalance between supply of co-factors necessary for DNA synthesis and demand for DNA production. The two co-factors which are the most important are folate and vitamin B12. When these are deficient, megaloblastic change results. On the other hand, increased demand for DNA in physiologically hyperproliferative states, such as cancer and hemolytic anemia, can cause megaloblastic change even in the face of freely available folate and B12. To understand why folate and B12 are so important to DNA synthesis, it is necessary to gird one's loins for a trip back to Biochem. DNA differs from RNA in that 1) deoxyribose is used instead of ribose, and thymine is used instead of uracil. The structural formulas below show that uracil and thymine differ only by a silly little methyl group.

Unfortunately, the methyl is necessary for the magic DNA enzymes to recognize the molecule as DNA and work their wonders on the double helix. It seems that the body can make just about any organic molecule out of yesterday's BK Broiler and a few trace metals, but in this case a special set of reactions is necessary for the finishing touches on thymine. To stick the methyl group on the ring, folate is required, and B12 helps out.

B. Folate

Just to make casual observers think we're studying real science here, let us look at the chemical structure of folate and its metabolites.

Note that "folate" and "folic acid" are basically the same thing and differ only in whether the carboxylic acid groups are dissociated, this in turn dependent only on ambient pH. Folic acid is a vitamin found in abundance in many foods, especially asparagus, broccoli, endive, spinach, and lima beans. The daily requirement is only 50 µg (Cf. 10,000 µg for iron). The body's folate reserves last about four months. The vitamin is rapidly absorbed by the proximal jejunum. Folic acid is metabolically inactive until it is converted into tetrahydrofolic acid (THF). A key enzyme in this conversion is dihydrofolate reductase, which is the target enzyme inhibited by the anticancer drug methotrexate. THF is capable of methyl group transfer by picking up the one-carbon group from the amino acid serine and sticking it on uridylate, thus producing thymidylate, which in turn goes off to seek its fortune in DNA as a courier of genetic messages. The methyl-carrying version of THF is called N5,N10-THF and is shown above.

As complicated as all this seems, it only scratches the surface. Folate has been shown to play a role in no fewer than six biochemical reactions, including synthesis of methionine, synthesis of purines (thymine is a pyrimidine), and catabolism of histidine. Failure of folate to break down histidine results in accumulation of an intermediary metabolite, formiminoglutamic acid (FIGlu), which can be measured in the clinical laboratory as a marker for folate deficiency.

C. Vitamin B12

Vitamin B12 is a substance whose
biochemistry is complex. Basically it consists of a cobalt-containing
porphyrin-like prosthetic group attached to a nucleotide (cobalt is a
major trace metal in human physiology, with the average body containing
1.1 grams of the element, third in abundance after iron and zinc).
The diagram at left shows the structure of cyanocobalamin, one the
simpler forms of the vitamin. B12 has
been called the most complicated molecule in the human body that is not
simply a repeating chain of smaller units (like proteins and nucleic
acids). The laboratory synthesis of this substance, not accomplished
until 1971, capped off the distinguished career of Nobel-winning
organosynthesist Robert Burns Woodward.

Deficiency of this vitamin produces megaloblastic anemia due to its role in folate metabolism. During the many transformations of folate from one form to another, a proportion gets accidentally converted to N5-methyl-THF, an inactive metabolite. This is called the "folate trap," since there is no way for active N5,N10-THF to be regenerated except through a reaction for which a form of vitamin B12, methyl-B12, is a cofactor (see diagram, right). Deficiency of B12 then produces a situation where more and more folate is trapped in an inactive form with no biochemical means of escape. The end result is failure to synthesize adequate DNA.

B12 deficiency also produces nervous system lesions not seen in folate deficiency. These lesions are manifest clinically as combined systems disease, a constellation of findings related to demyelination of axons in the spinal cord and cerebrum. These patients have decreased vibratory and proprioceptive senses in the extremities, spastic ataxia, disturbances of vision, taste, and smell, irritability, and somnolence. "Megaloblastic madness" is the term appended to the poorly documented cases of bizarre, sometimes psychotic behavior in B12-deficient patients. Clearly, there must be something that B12 is supposed to do about which folate has little or no concern. The best guess is that it has something to do with B12's role as a cofactor for methylmalonylCoA mutase, which catalyzes the following reaction:

A few of you, the truly perverted, will recognize this reaction as the clever way the body has of dealing with odd-chain fatty acids, prestidigitating them into even-chain species that can be handily dispatched via beta-oxidation. Without this mechanism, no telling what sorts of havoc these three-carbon residues would raise floating around in the myelin-making factory. A nice spin-off is that methylmalonic acid accumulates and shows up in the urine, where it can be measured as a marker of B12 deficiency.

The etiology of B12 deficiency is more complicated than that of folate deficiency. One can develop deficiency through either of the following mechanisms:

Dietary deficiency

B12 is only found in animal products, but it is plentiful. Therefore, nutritional deficiency is seen almost exclusively in vegans. Even so, this is extremely rare. Unlike the situation with folate, B12 body reserves can last for years.

Malabsorption states

By far, this is the most common mechanism of disease development. The absorption of B12 is much more complicated than that of folate and iron. B12 is absorbed only in the terminal ileum. Absorption occurs only if the B12 is bound to a glycoprotein, unimaginatively named intrinsic factor, produced only by the parietal cells of the gastric mucosa. Therefore, malabsorption can occur if 1) intrinsic factor is not produced, 2) intrinsic factor is neutralized, 3) there is a pathologic lesion of the terminal ileum, or 4) there are microorganisms present which compete successfully with the host for the B12. Let us consider each mechanism in turn:

Intrinsic factor not produced

This is seen following total gastrectomy. One must always give maintenance parenteral B12 for life following gastrectomy. Intrinsic factor is also not produced in pernicious anemia (see below).

Intrinsic factor inhibited

Pernicious anemia (PA) is the classic term used to describe the megaloblastic anemia which develops as a result of autoimmune destruction of the gastric mucosa (atrophic gastritis) and autoantibodies directed against intrinsic factor. PA is said to occur most commonly in elderly Caucasians of northern European extraction and in younger African-Americans. There is a strong association with other autoimmune diseases in the same patient, especially Hashimoto's thyroiditis.

Terminal ileum lesions

Crohn's disease is a classic example. It typically affects the terminal ileum and can produce malabsorption because of massive tissue destruction in this area.

Competition for B12

This occurs in various congenital or acquired anatomic abnormalities of the small intestine which foster overgrowth of our tiny little prokaryotic friends, all too happy to slurp up all that B12 at our expense. Another condition (a classic National Board question if there ever was one) is infestation with the fish tapeworm, Diphyllobothrium latum, also quite capable of quaffing a few cobalts.